1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device that includes a semiconductor element that produces heat during operation and a substrate including the semiconductor element mounted thereon.
2. Description of the Related Art
To efficiently allow heat produced by a semiconductor element included in a semiconductor device to escape, a substrate of low thermal resistance is used as a substrate for mounting the semiconductor element thereon. Examples of the substrate of low thermal resistance currently in practical use include a metal base substrate formed by a metal plate of high thermal conductivity, and high temperature co-fired ceramic (HTCC) substrates, such as an aluminum nitride substrate and an alumina substrate.
For example, Japanese Unexamined Patent Application Publication No. 61-212045 describes a technique in which a ceramic multilayer wiring substrate having a hole and a radiating fin (metal plate) having a protrusion are combined together, with the protrusion fitted in the hole, and a semiconductor element is mounted on the protrusion of the radiating fin (metal plate) to improve heat dissipation of the semiconductor element.
However, the technique described in Japanese Unexamined Patent Application Publication No. 61-212045 cannot be applied to the case where a semiconductor element to be mounted is, for example, a field-effect transistor (FET) that requires input and output of signals through its lower surface. This is because, if a plurality of semiconductor elements are directly mounted on the single radiating fin (metal plate), each of the semiconductor elements cannot input and output signals through its lower surface.
Japanese Unexamined Patent Application Publication No. 5-315467 describes a technique in which a second substrate of high thermal conductivity is disposed in a hole or notch of a first insulating substrate, and a semiconductor element is mounted on the second substrate to improve heat dissipation of the semiconductor element. With the technique described in Japanese Unexamined Patent Application Publication No. 5-315467, if an insulating substrate is used as the second substrate, a semiconductor element that requires input and output of signals through its lower surface can be mounted on the second substrate.
However, the technique described in Japanese Unexamined Patent Application Publication No. 5-315467 cannot fully support higher current that has been demanded in recent years. An FET that has been recently developed using SiC or GaN semiconductor has a heat resistance temperature as high as 300° C. or 500° C. Although high current can be supplied in this case, the technique described in Japanese Unexamined Patent Application Publication No. 5-315467 cannot fully support such high current. The reasons for this are as follows.
The insulating substrate used in the technique described in Japanese Unexamined Patent Application Publication No. 5-315467 is an HTCC substrate made of, for example, aluminum nitride or alumina having a sintering temperature of about 1600° C. For simultaneous firing with the insulating substrate, the material used to form a wiring conductor in the insulating substrate needs to be metal having a relatively high melting point, such as tungsten or molybdenum. However, metal having a relatively high melting point, such as tungsten or molybdenum, has a high electrical resistivity and is not suitable for higher current.
To realize supply of high current using metal having a high electrical resistivity, it is necessary to increase the cross-sectional area of the wiring conductor. This hinders downsizing of the semiconductor device.
In the case of metal having a relatively low electrical resistivity, such as silver or copper, high current can be supplied even when the cross-sectional area of the wiring conductor is relatively small. However, since the melting point of such metal is lower than the sintering temperature of the HTCC substrate, simultaneous firing with the HTCC substrate is not possible. If the wiring conductor is one that is to be added to the existing substrate surface, metal, such as silver or copper, may be used as a material of the wiring conductor. However, if the wiring conductor made of metal, such as silver or copper, is formed only on the substrate surface, the resulting increase in substrate area leads to an increased size of the semiconductor device.
The same applies to the technique described in Japanese Unexamined Patent Application Publication No. 61-212045. Since an alumina substrate is used as the ceramic multilayer wiring substrate described in Japanese Unexamined Patent Application Publication No. 61-212045, metal of low resistivity, such as silver or copper, cannot be used to form the internal wiring conductor.